At present, switching power supplies are used extensively and replace linear regulated power supplies, but a switching power supply must rely on a pulse width modulation (PWM) controller to control the ON/OFF time of a power transistor. The PWM controller outputs a voltage or a current based on the output voltage/current and power to generate each feedback signal to modulate the pulse width outputted by the PWM controller to produce appropriate ON/OFF time. A conventional switching power supply usually cannot meet the requirements of different output voltage regulation very well by a primary-side feedback method, and thus most feedback signals of switching power supplies must be obtained by a secondary side and an optical coupler transmits a secondary-side feedback signal to the primary side to control the output pulse signal of the PWM controller. Therefore, feedback and current limit components including the photo-coupler, shunt regulator, current limit resistor and operational amplifier must be used in the switching power supplies.
Referring to FIG. 1 for a circuit block diagram of a conventional primary-side feedback switching power supply, a circuit is started to supply a current from a DC input power source VIN to a resistor R2 to charge a capacitor C3. If the source voltage VCC reaches a voltage large enough to start the PWM controller U1, the PWM controller U1 will start outputting a pulse wave to control a power transistor Q1. If the power transistor Q1 is conducted electrically, a current will be supplied from the DC input power source VIN and passed through a primary-side coil of a transformer T1 and the transistor Q1 and then returned to a negative terminal of the input voltage VIN. Since a diode D3 has a polarity opposite to that of the output winding on the secondary side, therefore the energy cannot be transmitted from the primary side to the output terminal, but the energy is temporarily stored in the transformer T1 instead. If the primary-side current detects that the voltage of a resistor R6 has reached a voltage reference value, then the power transistor Q1 will become OFF. At the time, the polarities of both diode D3 and output winding become positive on the secondary side, and thus the energy stored in the transformer T1 will be transmitted to an output terminal VO, and an output capacitor C4 will be charged for several cycles to an output voltage regulated point. On the other hand, the voltage of a primary-side auxiliary winding provides a current to the supply source voltage VCC via a resistor R1 and a diode D1, and supplies the current to the PWM controller U1. In another path of the auxiliary winding, the current is passed through the resistor R1 and diode D2 and rectified with a capacitor C6 to become a DC voltage, and divided by the resistor R3, R4 and sent to a voltage feedback input pin VFB of the PWM controller U1. The voltage of a capacitor C6 will reflect a change of the output terminal VO. Since the voltage of the output rectified diode D3 will drop based on the change of a nominal load and the voltage of the capacitor C6 will be affected by the duty cycle, therefore the voltage detection method by using the voltage of the primary-side capacitor C6 cannot fully reflect the variation of the output voltage VO of the secondary side. Therefore, the voltage regulation of the output voltage cannot provide a more stable voltage output. In addition, the output terminal is usually connected to a dummy load resistor R8 to prevent the output voltage drift to high limited potential at a light load condition, because it requires the connection of a dummy load resistor R8. As a result, the requirement of the regulation specified by the GREEN MODE that requires having an input power of less than 0.3 W at no load cannot be satisfied. In the meantime, this method does not provide an output current limit circuit, and thus the output current will increase when the output voltage decreased. A circuit protection depends on the primary-side current to detect a feedback signal of the resistor R6 to limit the output power. If the supply source voltage VCC is lower than the minimum working voltage of the PWM controller U1 and the PWM controller U1 is turned off, the PWM controller U1 will restart again after it is charged by the resistor R2 to the start voltage level, so that the overall output current and input power can be reduced effectively.
Referring to FIG. 2 for a circuit block diagram of a conventional secondary-side feedback switching power supply, the voltage at the output terminal VO is divided by resistors R8, R9 and inputted into the input terminal of a feedback voltage regulator U3, and the output terminal of the shunt regulator U3 controls a secondary-side LED current of a photo-coupler U2 that is converted into a primary-side current signal for controlling the voltage of a voltage feedback input pin VFB of the PWM controller U1. Under a stable output load, the voltage of the voltage feedback input pin VFB of the PWM controller U1 is constant. If the output load is changed, the voltage of the voltage feedback input pin VFB will be regulated to maintain a stable voltage output. Since the voltage exceeds the supply source voltage VCC, the primary-side auxiliary winding starts charging the capacitor C2 and supplies the voltage required by the supply source of the PWM controller U1. In FIG. 2, an output current limit circuit is formed at the secondary side by resistors R5, R6, R7, capacitor C4 and transistor Q12. If the voltage of the resistor R6 exceeds the voltage of a base-emitter junction of the transistor Q12, the current limit starts operating. The collector of the transistor Q12 controls the secondary-side LED current of the photo-coupler, so that the pulse width of the primary-side PWM controller U1 is restricted, and the output current limit may vary easily with the temperature drift due to the variation of base-emitter junction voltage, and thus the precision of the current limit operation will be low and is applicable for a low-price changer circuit only.
Referring to FIG. 3 for another circuit block diagram of a conventional secondary-side feedback switching power supply, the precision here is very high and applicable for a switching power supply with lager power and larger output current. Operational amplifiers U4A, U4B, a shunt regulator U3 and a photo-coupler U2 are used for the voltage feedback and output current limit circuits. The shunt regulator U3 is a voltage reference generator for providing a stable 2.5 V voltage reference to the voltage feedback operational amplifier U4A and the current limit operational amplifier U4B, the output terminals of the operational amplifier U4A and U4B to connect to a diode D3, D4, and then connect to a secondary-side LED terminal of the photo-coupler U2, for controlling the output pulse width of the primary-side PWM controller U1 to achieve the voltage regulation and current limit functions. This circuit is used very often due to its high precision, but it has the drawbacks of using many components and incurring a high cost.
Therefore, it is a subject of the present invention to design and develop a primary-side feedback switching power supply capable of maintaining a constant output current when the output current is restricted and the output voltage is dropped.